Background: The purpose of this study was to examine the relations between children's social competence and initial index of theory of mind at 30 months of age. Methods: The participants of the study were 322 toddlers and parents/caregivers who were registered with the Japan Science and Technology Agency (JST) project. They completed a five-minute interaction session, which was coded using the Interaction Rating Scale (IRS) as an evidence-based practical index of children's social competence. In addition, the children were asked to complete a diverse-desire task as a ToM (theory of mind) index. Results: The results showed that the ToM index was related to the total score and subscales of the IRS, such as Empathy and Emotional regulation. Conclusions: These findings show that the IRS score was related to ToM task performance at 30 months of age.
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